Brake system with a new type of MUX control (MUX 2.0), having an outlet valve per brake system or an outlet valve per brake circuit, and method for controlling pressure

ABSTRACT

A brake system for motor vehicles may include an actuation device (e.g., brake pedal), a travel simulator to generate a feedback force on the actuation device, a first piston-cylinder unit having at least one piston that separates two working chambers that are connected via at least one hydraulic line to at least one wheel brake of a brake circuit, a control device and a pressure supply unit driven by an electric motor. At least one wheel brake may be assigned to each brake circuit, and each wheel brake may be connected to its associated hydraulic connecting line via a controllable switching valve. An outlet valve may be assigned to a single wheel brake or to a single wheel brake of each brake circuit in a hydraulic connection between the wheel brake and a pressure medium storage container, without any further valve disposed as such.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 of International Application No. PCT/EP2015/081401, filed Dec. 30, 2015, which was published in the German language on Sep. 22, 2016 under International Publication No. WO 2016/146222 A3, which claims priority under 35 U.S.C. § 119(b) to German Patent Application No. 20 2015 107 079.9, filed Mar. 16, 2015, German Patent Application No. 20 2015 107 075.6, filed Mar. 16, 2015, and German Patent Application No. 20 2015 107 072.1, filed Mar. 16, 2015, the disclosure of which are incorporated herein by reference in their entirety.

The invention relates to a brake system for a motor vehicle.

PRIOR ART

WO2006/111392A1 and WO2010/091883 A1 disclose brake systems in which, in ABS mode, the pressure in the wheel brakes is adjusted simultaneously or successively in a closed multiplex process. This takes place via a switching valve and via the position-regulated control of a driven piston for pressure build-up and pressure reduction taking into account the pressure-volume curve of each individual wheel brake. Preferably, switching valves with low flow resistance are used in connection to the wheel brake. The pressure can here be changed in one or more wheel brakes sequentially or simultaneously. For control, a pressure sensor is used which measures the pressure in the hydraulic connection between the piston-cylinder unit and wheel brakes.

The advantage of this method is a very precise pressure regulation, in particular with low friction coefficients and in recuperation. Also, the cost of the valves can be significantly reduced, since only one switching valve is required instead of one inlet and one outlet valve per wheel brake. The disadvantage with the brake systems known from these two documents is the high requirements for the electric motor. This must amongst others have a low inertia mass and a high torque for reversed operation.

DE 10 2012 002 791 A1 discloses a wheel brake, the basic structure of which with a master brake cylinder and isolating valves is known in the market as MKC1, see also DE 10 2013 224313 A1. Multiplex mode here is configured such that both the master brake cylinder and the pressure supply unit are connected to the brake circuits via respective isolating valves.

The advantage of this arrangement is the modular structure and the use of standard components (master brake cylinder), and the use of a separate pressure supply unit. In this arrangement, no differential pressures occur in the brake circuits since the pressure supply unit is connected to the brake circuits via isolating valves, and no differential pressures occur due to the interconnection of a piston for media separation.

The disadvantage however is the high cost of components. Amongst others, a large number of valves is required, with a complex master brake cylinder with two chambers and a simulator.

DE 102014117727 supplements the brake system described in DE 10 2012 002 791 A1 with a novel pressure supply unit which has a double-stroke piston operated in advance and return strokes, and in advance stroke has a different hydraulic cross-section area than in return stroke, and with which a controlled pressure reduction is possible.

The advantage of this configuration is the continuous delivery by the pressure-generating unit, and the motor-downsizing potential in conventional brake systems with inlet and outlet valves, by the use of a smaller hydraulic area. Because of the high dynamic requirements for motor torque for use in regulating mode, the advantage of downsizing cannot however be utilised and hence the brake system cannot be advantageously minimised.

Furthermore, various master brake cylinder designs with travel simulator are known from the prior art which are constructed with two pistons or three pistons and a travel simulator.

Advantageous designs of the master brake cylinder provide corresponding valve circuits for the travel simulator (shut-off in fall-back level, function valves, infeed on fall-back level), and isolating valves for the brake circuits for decoupling the pedal in brake-by-wire mode. Reference is made here merely as an example to DE 10 2010 081463 A1 and DE 10 2013 216477 A1. Further brake systems are known from DE 10 2015 103859.5 (MUX with two pistons), DE 102011102270 (three pistons), DE 10 2013216477 A1 (CAS, 3-piston system, auxiliary piston, push-rod piston, floating piston with partial MUX mode) and DE 10 2013224313.

The pressure regulating module described in this invention functions with all the above-mentioned designs for brake-by-wire master brake cylinders with travel simulator, and is therefore not explained in more detail below. Differences in the master brake cylinder design occur substantially because there are different customer preferences with regard to pedal feedback, and automotive suppliers wish to use standard components in the master brake cylinder, and certain designs require one or more isolating valves for the pressure supply unit.

DE10 2013 216477 A1 describes a three-piston THZ with valve circuit for the pressure supply unit and pressure regulation for ABS. In normal mode, the second pressure chamber is pressureless and the third is assigned to the SK piston. This locks in its starting position. The pressure regulation takes place in the HA circuit with MUX, and in the front axle circuit either with MUX or two additional outlet valves which, in P_(ab) function, conduct the pressure medium to the storage container via an additional valve. Pressure regulation in multiplex mode does not take place via the volume measurement known from WO2006/111392A1, but via PWM of so-called wheel valves with constant pressure measurement by means of pressure sensors.

OBJECT OF THE INVENTION

The object of the present invention is to provide an economic brake system with high regulation quality and regulation performance.

ACHIEVING THE OBJECT

The object of the invention may be achieved with a brake system with features according to various ones of the attached claims.

According to one aspect of the present application, a brake system for motor vehicles may include an actuation device, a travel simulator configured to generate a feedback force on the actuation device, at least two brake circuits, each respective brake circuit including at least two wheel brakes, wherein each respective one of the at least two brake circuits includes one respective outlet valve associated with one of the wheel brakes of the respective brake circuit or wherein only one outlet valve is associated with the at least two brake circuits, and wherein the outlet valve is, or the respective outlet valves are, arranged to couple the brake circuits to a pressure medium storage container only for pressure reduction, and with no further valve arranged between the outlet valve, or the respective outlet valves, and the pressure medium storage container, a first piston-cylinder unit, having at least one piston and at least one working chamber, wherein the at least one working chamber is coupled via at least one hydraulic connecting line to at least one of the wheel brakes of the brake circuits, at least one pressure supply unit comprising an electric motor, a piston arranged to be driven by the electric motor, and at least one working chamber, wherein the at least one pressure supply unit is able to simultaneously reduce brake pressure in two or more of the wheel brakes, respective switchable valves connected in respective hydraulic connecting lines coupled to respective wheel brakes of the brake system, wherein the respective switchable valves are open when unpowered, wherein the respective switchable valves are used in both pressure build up and pressure reduction in one or more of the respective wheel brakes, and wherein the respective switchable valves are arranged to conduct pressure medium between the respective wheel brakes and the first piston-cylinder unit, via the hydraulic connecting lines, and between the respective wheel brakes and the at least one pressure supply unit, a control device, coupled to control at least the switchable valves and the outlet valves, and at least one isolating valve, a respective one of the at least one isolating valve being disposed in a respective one of at least two further hydraulic connecting lines between at least one of the at least two brake circuits and the at least one pressure supply unit, wherein the at least one isolating valve isolates the at least one of the at least two brake circuits from the at least one pressure supply unit when closed, wherein each of the respective switchable valves includes an interior or armature housing, a first inlet/outlet port, and a second inlet/outlet port, wherein the second inlet/outlet port is a valve seat inlet/outlet port, wherein the interior or armature housing of each of the respective switchable valves is connected, through the respective first inlet/outlet port of the respective switchable valve, via a part of the respective hydraulic connecting line, to a pressure supply unit of the at least one pressure supply unit and to a brake circuit, and wherein the valve seat inlet/outlet ports of the all of the respective switchable valves are connected via respective parts of the respective hydraulic connecting lines to respective associated wheel brakes, and wherein the respective switchable valves are enabled to be opened both by return springs thereof and by pressure in the respective wheel brakes.

The brake system according to the invention is distinguished by a significant improvement in comparison with previously known brake systems with multiplex regulation, which are configured with 4 switching valves or 8-valve technology with inlet and outlet valves for ABS. The brake system according to the invention can advantageously be used universally in combination with different designs of master brake cylinder of a brake-by-wire system.

The brake system according to the invention is distinguished by a highly dynamic MUX mode and allows a great improvement in performance and great reduction in cost due to the minimal number of valves, wherein advantageously simple switching valves based on modified inlet valves may be used. Also, only a few pressure emitters/sensors are required. It is of particular advantage that only a small low-cost motor is required as a drive for the pressure supply unit.

The invention advantageously provides a pressure regulating module with pressure regulation which is distinguished by high pressure regulation quality, high dynamics thanks to short cycle time, particularly simple design of the low-flow switching valves with advantageous contact flow, a reduction in the requirements for the motor of the pressure supply unit, and minimal flow resistance.

This is achieved with a compactly constructed brake system which expands the advantages of a multiplexer in the form of high regulation quality in pressure regulation by pressure-volume control in various operating modes such as e.g. recuperation, ABS, ESP, ASR, by novel pressure reduction and pressure build-up regulation concepts which require few outlet valves to reduce the cycle time in the temporarily open brake circuit.

The requirements imposed are fulfilled according to the invention by operation in the closed and partially open brake circuit with minimal number of valves, and preferably an advantageous design of the pressure-generating unit with intelligent construction of the pressure-generating unit with a pressure piston delimiting only one working chamber, or a double-stroke piston which delimits two working chambers.

The following basic concepts form the background to the brake system according to the invention:

-   -   pressure regulation in the closed and partially open brake         circuit with minimal volume loss in ABS regulation mode;     -   pressure build-up and pressure reduction with many degrees of         freedom, high regulation quality and dynamics with switching         valves and only one outlet valve in just one or both brake         circuits;     -   partially simultaneous pressure build-up and pressure reduction         in the closed brake circuit using a double-stroke piston in the         pressure supply unit;     -   use of current measurement of the electric motor of the pressure         supply unit for indirect pressure measurement, and use in         particular in the pressure reduction and pressure build-up         regulation in two brake circuits;     -   more precise pressure-controlled pressure reduction by means of         pressure measurement via pressure sensors via the pressure         supply unit and valves which connect the pressure supply unit to         the storage container (double-stroke piston with PD1, PD3         valve),     -   novel design of the pressure supply unit in the embodiment as         double-stroke piston with pre-fill effect, use of different         hydraulic surface areas in particular on pressure build-up at         high pressures;     -   minimising of operation in the open brake circuit by preferably         regulation in multiplex mode in the closed brake circuit.

In the brake system according to the invention, evidently not all the above-mentioned ideas need be implemented, but it is essential that only a maximum of one outlet valve is provided per brake circuit, and one switching valve per wheel brake for pressure build-up and pressure reduction, whereby advantageously the number of necessary valves is less than the eight which are required with a conventional ABS system.

The brake system according to the invention also has a novel intelligent multiplex method which provides a largely simultaneous pressure reduction in several wheel brakes via time control of the outlet valve or valves, and optionally allows also simultaneous pressure reduction and pressure build-up in different brake circuits.

The invention is based on the concept that in normal operation with low dynamic requirements and high pressure-setting precision requirement, in particular with normal brake force amplification, recuperation, ABS at low μ, in all wheel brakes or wheel brake cylinders, the pressure is built up and reduced simultaneously or sequentially via travel control of the piston of the pressure supply unit, taking into account the pressure-volume curve(s). Here no PWM control of the switching valves is used, or a simplified valve circuit. Instead, the switching valves assigned to the wheel brakes on pressure setting are always open for the entire time and closed after reaching the desired or predefined nominal pressure, in order to maintain the brake pressure in the wheel brakes. In operating situations with high dynamic requirements such as e.g. ABS at high μ, μ split, ESP and ASR, the pressure in principle is always built up in all wheel brake cylinders with pressure-volume control in multiplex mode, i.e. simultaneously or sequentially. Here again, there is no PWM control of the switching valves, but the pressure is reduced in some of the wheel brakes in multiplex mode simultaneously or sequentially, while in the one or both wheel brakes the pressure is transferred to the storage container via the respective assigned outlet valve, wherein the respective outlet valve is opened only for a predefined time so that during this time, the pressure in the wheel brake can fall to the nominal pressure. Also, the pressure reduction can take place via a working chamber of the pressure supply unit, and from there via a switching valve to the storage container. The switching valve is also time-controlled so that in the predefined time in which the valve is open, the pressure can fall to the nominal pressure. The pressure reduction in the other wheel brakes may take place at the same time via the volume control by means of the piston of the pressure supply unit.

The pressure fall regulation in multiplex mode in the closed brake circuit is extended in comparison with the prior art in that, on simultaneous pressure reduction in two wheel brakes, the switching valves SV are open simultaneously or with a temporal offset, wherein the switching valve of the wheel with the higher pressure is opened early.

The pressure is reduced in the open brake circuit preferably by time control of the outlet valves to the storage container. By pressure reduction via outlet valves, the brake circuit is opened for a short-time.

Thanks to the above extensions, the load on the multiplexer or the pressure supply unit can be greatly reduced and at the same time the regulation quality increased thanks to shorter cycle times.

Thus, the pressure in one brake circuit can be reduced rapidly in that the pressure is reduced by opening the outlet valve of the one wheel brake, and at the same time the pressure in the other wheel brake of the brake circuit is reduced by means of the pressure supply unit. In a conventional multiplexer without corresponding outlet valve, the pressure reduction in the two wheel brakes of a brake circuit would have to take place temporally successively, and thus take at least twice as long.

Also, advantageously the outlet valve can be used for the pressure reduction in both wheel brakes of the brake circuit if both the outlet valve and the two switching valves assigned to the wheel brakes are opened in multiplex mode.

Advantageously, only one outlet valve is used in a brake circuit, in particular to simplify the regulation in brake circuit II. Thus, one outlet valve (AV3, FIG. 1 b ) is used to reduce the pressure in one or two wheel brake cylinders. For reduction in two wheel brakes via one outlet valve, the wheel brake cylinder is isolated from the pressure supply unit. At the same time, the pressure in brake circuit BK I and BK II can be built up or reduced simultaneously or sequentially by pressure-volume control of the wheel brakes. This degree of freedom leads to a significant reduction in the cycle time of the pressurisation of four wheel brakes, and has a highly advantageous effect on the regulation quality (deviation of wheel speeds from vehicle speed) in particular in extreme situations, e.g. changing μ, in particular in the high μ range, and leads to shorter braking distances. Also, the volume loss in the brake circuit is minimised because in regulation mode, only very little volume is lost. This has advantageous effects on a small dimensioning of the volume of the pressure supply unit.

The proposed full MUX systems of the prior art are known to have the problem of simultaneous pressure fall P_(ab) when the pressure level in the wheel brakes differs greatly. Many use the volume control to control the pressure by using the pressure-volume curve of the wheels/wheel cylinders. The time loss on pressure fall p_(ab) of the wheel/wheel brakes, in particular on the front axle, should however be as low as possible. The switching times of the known MUX systems however cause a significant time shift due to the switching time required in multiplex mode. Because of the high brake force proportion on the front axle (V), this is particularly susceptible to good regulation, which means that the wheels must always be operated with high dynamics and almost optimal brake pressure close to the slip optimum. The brake system according to the invention meets these demands particularly well if the high dynamic regulation strategy described above with volume and time control is used.

Furthermore, a (minor) disadvantage with the previously known brake systems described above is that no pressure build-up p_(auf) can take place if one wheel required a pressure fall p_(ab). As an alternative to full MUX, partial MUX systems are proposed in which one BK is designed with MUX and the other BK with conventional inlet and outlet valves.

One essential disadvantage of the outlet valves (AV valves) is the poorer precision of the pressure control, pressure fluctuations and noise formation. With the regulation strategy described above, primarily the quiet volume-controlled multiplex mode is used. The outlet valves are not required continuously and are used relatively rarely.

This is achieved by priority control of the multiplexer with first priority for the front axle and additional use of the outlet valve AV in unchoked return to the storage container. For the rear axle, the pressure reduction p_(ab) takes place with second priority in the multiplexer. Alternatively, a pressure fall p_(ab) may take place on the rear axle with precise time control of the switching valves, whereby only a slight time delay occurs.

Throughout the entire regulation process, at the start of the pressure reduction, the pressure in all wheels is stored so that the MUX control system of the pressure supply unit (DE) can immediately switch to optimal control pressure. Precise time control of the outlet valves, in comparison with the prior art, is possible because the pressure difference is known, and from the pressure-volume curve the volume and hence the throughflow quantity for time control of the outlet valve or a switching valve can be determined. Pressure fluctuations towards the end of the pressure change can also be reduced by supporting the pressure reduction with a corresponding piston control of the pressure supply unit.

The temporal sequence of the method is shown and explained later in detail in the figures.

The term volume control according to the invention means that the control device evaluates the current pressure levels, the pressure-volume curves and the target nominal pressures for the respective wheel brakes, and using these data calculates the necessary delivery volume which must be supplied by the pressure supply unit. From this delivery volume, the necessary movement travel of the piston of the pressure supply unit can be determined. With corresponding valve circuits and design of the pressure supply unit, it is possible to implement a pressure build-up P_(auf) in one brake circuit and at the same time a pressure reduction p_(ab) in another brake circuit.

Previously, the valves required for multiplex operation were more costly because of the requirements for differential pressure and flow cross-section due to the large dimensioning of the magnet circuit. By corresponding supply to the magnetic valve from the brake circuit into the armature chamber, and then via the valve seat to the wheel cylinder, with the brake system according to the invention, advantageously, a low-cost standard magnetic valve can be used.

To support the regulation, a pressure sensor is used to determine the pressure in one brake circuit. The pressure in the other brake circuit can be determined indirectly on separation by an isolating valve, using the known method of phase current measurement of the electric motor. The accuracy of the pressure estimate is increased further if a temperature sensor is arranged in the electric motor driving the piston, since the torque constant changes proportionally to the temperature. With the known cross-section area of the master brake cylinder and gear reduction, the pressure can be calculated by the proportional correlation between the phase current and torque of the electric motor.

An advantageous feature of the brake system according to the invention is the use of a double-stroke piston with which the pressure can be built up and reduced in multiplex mode. It is also advantageous if a further valve (TV2 b, or ZAV) is available for pressure reduction in the second chamber (4 a, FIG. 5 ) of the double-stroke piston. In particular with the double-stroke piston, the use of one or more pressure reduction valves (PD3, PD1 FIGS. 5 a-5 d , FIG. 6 ) is useful, so that the pressure in the brake system can be largely reduced in closed mode and pressure reduction can take place with low noise even at high pressures. This is desirable in particular in brake servo mode with fading and pressure reduction when stationary after ABS intervention.

The pressure is then reduced either via the piston return stroke, the pressure-regulated pressure fall by means of pressure measurement via the pressure sensor for the pressure supply unit, and/or via valves which connect the pressure supply unit (double-stroke piston) to the storage container (i.e. PD3, PD1). The pressure sensor in brake circuit BK II is used to regulate the pressure reduction in both brake circuits. If the pressure is to be reduced individually in brake circuits I and II, in addition the pressure estimate based on the phase current measurement is used. The pressure reduction can here take place via one chamber 4 of the piston or both chambers 4 and 4 a.

In normal brake servo mode, the pressure is reduced via the piston return stroke to pressures close to the blocking pressure, the pressure fall via PD3, PD1 on pressure reduction from high pressures, in particular after fading or at the end of ABS control processes.

The double-stroke piston in the pressure supply unit may be configured such that the hydraulic surface areas in the advance and return strokes are different. By changing the hydraulically active areas, the torque requirement at high pressures is reduced. At the same time, a pre-fill effect can be achieved, i.e. by a larger volume flow at low pressures, a very rapid brake application can be achieved or a pad clearance overcome.

The small hydraulically active area is effective because the double-stroke piston is operated in return stroke, or additionally, in advance stroke, the front and rear chambers of the double-stroke piston are connected via a changeover valve (ShV) or two valves (TV2 and TV2 b), and hence a small hydraulic area acts in pressure build-up. If the double-stroke piston is retracted, by opening a pressure reduction valve PD1, the pressure in both brake circuits can be dissipated into the storage container. Thus low-noise operation in closed brake circuit is possible. By intelligent activation, the opening of the isolation valve can be supported even at high differential pressures (brake circuit pressure to pressure in the double-stroke piston) if the double-stroke piston changes the pressure in the working chamber before opening of the valve, and allows opening at low differential pressures. This allows a downsizing of the isolating valve, in particular its design for high throughflow and low differential pressures.

The pressure reduction then takes place either via the piston return stroke (PD1 open) via pressure-volume control and if necessary opening of outlet valves in brake circuit II and pressure reduction via TV2 b (ZAV). For noise reduction, an outlet valve can be opened time-controlled for this operating point and the pressure fall influenced by the piston, so that pressure fluctuations are avoided and a gentle swing towards the target pressure level is achieved. This may be used effectively in particular on pressure fall via ZAV.

In the return stroke with valve PD1 closed, pressure can only be built up or the volume of one brake circuit shifted to the other. This pressure build-up is preferably used only if the pressure must be raised significantly above the normal operating level, such as e.g. on fading >120 bar.

Also, pressure can be reduced in one brake circuit in one or two wheel brake cylinders, and at the same time pressure built up in the other brake circuit with the pressure-volume control method as depicted in FIG. 5 b . The pressure is regulated via the correspondingly adapted pressure-volume curve, which takes account of volumes of the activated wheel brakes and the hydraulically active area. When the target pressure is reached, the associated valve is closed and the volume of the wheel brake still active is reduced via PD1. At the same time, a pressure reduction is possible via an outlet valve.

In the system, preferably MUX regulation is used, i.e. pressure control via the pressure-volume curve with closed brake circuit (FIGS. 2 a-2 b ). Thus, in the closed brake circuit, the pressure can be built up and reduced on the basis of the pressure-volume curve. This takes place mainly with brake force amplification, recuperation, ABS mode in low frequencies and pressure amplitudes. In other operating cases, e.g. controlled pressure fall after ABS operation, simultaneous pressure build-up and pressure reduction at high frequencies, the pressure can be influenced in pressure reduction in addition to time control of the outlet valves, if necessary supported by plunger travel control (FIG. 6 b ).

After the pressure reduction on opening of the brake circuit, there is always a volume loss in the brake circuit and hence a change in travel in the piston position of the pressure-generating unit. Therefore, it is useful to detect the offset shift Δs_(o) of the pressure-volume curve (FIG. 2 b ). This is not necessary for MUX regulation, but for regulation and optimising of the volume balance in order to prevent the piston from travelling to a stop on a regulation process. In particular on use of a double-stroke piston with limited volume in one stroke direction, information on the absolute position of the piston is important for regulation.

This proposed control with outlet valve(s) and pressure reduction p_(ab) time control substantially relieves the load on the motor dynamics. The rare use of the outlet valve brings the advantages that the ABS pressure modulation does not necessitate opening of the brake circuit, which lowers the probability of a brake circuit failure and entails particular advantages for autonomous driving/braking.

The pressure regulation module with pressure regulation and its various embodiments thus offers a module for perfect pressure regulation without restriction and high safety on faults. The disadvantages of the conventional multiplexer—e.g. long cycle time from sequential wheel operation, no possibility of simultaneous pressure build-up and reduction, high requirements for electric motor dynamics—are thus eliminated and form the basis for an almost perfect regulation with minimum valve complexity. Depending on choice of pressure supply unit (single-stroke piston or double-stroke piston), different degrees of freedom are possible. The single-stroke piston has the advantage of low software complexity; the double-stroke piston offers all degrees of freedom and motor-downsizing potential. Also, independently of the choice of pressure supply unit, the requirements for motor torque for reversed operation of the multiplex regulator are drastically reduced and the size and cost of the electric motor can be significantly reduced.

A further improvement in the system layout advantageously results from infeed of the volume of the pressure supply unit via a blow hole into the front side of the floating piston. This also advantageously increases safety with simultaneously reduced cost. With this system layout, the isolating valve TV1 may be omitted since the pressure supply unit is isolated via movement of the piston on system failure. This offers cost advantages (fewer valves) and reduces the flow resistance between the pressure supply unit and the first brake circuit (BK1).

DESCRIPTION OF THE DRAWINGS

FIG. 1 a : shows a first possible embodiment of the brake system according to the invention with master brake cylinder, pressure supply with outlet valve(s) in one or two brake circuits;

FIG. 1 b : shows an example of a simplified pressure-volume curve;

FIG. 1 c : shows pressure regulation options in the basic system of FIG. 1 a;

FIG. 1 d : shows valve circuit with AV/EV in the regulation systems of the prior art;

FIG. 1 e : shows advantageous valve circuit for new regulation system with switching valves and an outlet valve in one brake circuit;

FIG. 1 f : shows inlet valve according to the invention in the brake circuit;

FIG. 2 a : shows pressure-volume control in a closed brake circuit (AV, ZAV closed);

FIG. 2 b : shows pressure-volume curve for wheel brake 1 and wheel brake 2 with offset shift by opening of brake circuit;

FIG. 3 : shows conventional multiplex regulation in sequential order;

FIG. 3 a : shows cycle shortening of multiplex regulation with AV valve in pressure reduction;

FIG. 3 b : shows time control on pressure reduction via outlet valve;

FIG. 4 : shows temporal development of an exemplary regulation with 4 wheel brakes;

FIG. 4 a shows temporal development of an exemplary regulation with 4 wheel brakes;

FIG. 4 b : shows temporal development of an exemplary regulation with 4 wheel brakes;

FIG. 5 : shows advantageous brake system structure with double-stroke piston (DHK);

FIG. 5 a shows pressure build-up regulation in multiplex mode according to the invention with DHK and outlet valve;

FIG. 5 b shows simultaneous pressure build-up and pressure reduction in multiplex mode in DHK return stroke and outlet valve;

FIG. 5 c shows simultaneous pressure build-up and pressure reduction in multiplex mode in DHK advance stroke and outlet valve;

FIG. 5 d shows regulated or controlled pressure reduction in both brake circuits in closed circuit via pressure-generating unit and PD3 valve;

FIG. 6 : shows double-stroke piston system in advantageous dual circuit design.

DESCRIPTION OF THE FIGURES

FIG. 1 a describes the basic embodiment of the brake system according to the invention with master brake cylinder HZE, pressure supply unit DE with single-stroke piston (3) and outlet valve(s) AV1, AV3 in one or optionally two brake circuits. Brake circuit II is advantageously assigned to the front axle. The outlet valve AV1 is optional, i.e. need not necessarily be provided.

The brake system consists of a master brake cylinder according to the prior art, comprising a master brake cylinder unit HZE, floating piston SK with return spring 1, a pressure piston DK or ram or an auxiliary piston HiKo, a hydraulically actuated travel simulator WS and corresponding control valves HZV for the function of the piston-cylinder unit, as described for example in the prior art.

The following embodiments amongst others are possible:

-   a) master brake cylinder with two pistons in the form of a pressure     piston DK and a floating piston SK with connected travel simulator     which can be shut off via a valve, -   b) 3-piston system with auxiliary piston HS for travel simulator     actuation and infeed valve and/or mechanical intervention in the     event of a fault, -   c) 2-piston system with floating piston SK and auxiliary piston HiKo     with infeed.

In all embodiments, the master brake cylinder unit HZE can be isolated from the pressure supply unit DE. According to variant Var2, this can be achieved via isolating valves TV1 and TV2, or in the second variant Var1 shown, via blocking of the supply of the floating piston. The valve circuit of the HZE ensures that no undesirable feedback occurs to the pedal BP when the pressure supply unit DE is active, and in fall-back level (system failure) the volume of the master brake cylinder unit HZE is guided to the wheel brakes RB1-4. Also, a switching valve SV1-4 is arranged for each wheel brake in the hydraulic connection to the respective associated working chamber A1 or A2 of the brake master cylinder HZE. The concrete embodiment of the master brake cylinder HZE is not however relevant for the brake system according to the invention.

The brake system has four switching valves SV1, SV2, SV3 and SV4, via which the pressure supply DE and the master brake cylinder HZE are connected to the wheel brakes RB1-4. The switching valves SV1-4 preferably have a low flow resistance and are suitable for MUX operation. In addition, an outlet valve AV3 is provided in a brake circuit for pressure reduction in the wheel brake in RB3 independently of the MUX, and is arranged in the hydraulic connection between the wheel brake RB3 and the storage container 10. Preferably, the outlet valve AV3 is positioned on the front wheel brake RB3 of a brake circuit since, in extreme cases, the pressure in this wheel brake must be reduced quickly and without great time delay because the significant braking effect originates from the front axle.

The pressure supply unit DE consists of an electric motor M, which via a spindle 2 drives a piston 3 which compresses or shifts the volume in the pressure chamber 4. The motor M of the pressure supply unit may comprise two or three sensors: a) angle sensor 6, b) current measurement sensor for measuring the phase currents of the electric motor 7, and c) if necessary, a temperature sensor 8 for determining the coil temperature of the electric motor M.

The pressure-generating unit DE is preferably arranged in the valve block or HZE. The pressure chamber 4 of the pressure-generating unit DE is connected to a storage container 10 via a check valve 5. A pressure sensor 9 is arranged at the outlet of the pressure-generating unit DE. The brake circuit II is connected via the isolating valve TV2, and brake circuit I via the isolating valve TV1, to the pressure supply unit DE. The isolating valve TV1 may be omitted if one chamber is separated by the pressure supply unit DE in fall-back level. This can be achieved by a pressure infeed from the pressure-generating unit DE via the blow hole SL of the floating piston SK.

For pressure modulation in ABS and recuperation, the control device and its regulator determine the necessary pressure change for pressure build-up (referred to below as P_(auf)) and pressure reduction (referred to below as P_(ab)). The pressure is regulated by the pressure-generating unit DE, in that simultaneously or with a temporal offset, the individual wheels/wheel cylinders are supplied with pressure. For this, the electric motor M shifts the corresponding volume for pressure change in both directions via e.g. the piston 3.

Here, the pressure change according to the prior art can be modified by corresponding time control with PMW of the switching valves and pressure control of the pressure of the DE. This however requires a very precise PWM process with complex pressure model. Preferably, therefore, the volume control is used as already described above. For this, the data of the pressure-volume curve (p-V curve—see FIGS. 1 a and 2 a ) of the respective wheel brake RB1-4 involved in the pressure build-up or reduction are stored in the memory of the regulator unit. If now the regulator requests a pressure change Δp, for pressure regulation at the wheel the differential volume ΔV is adjusted accordingly by the piston in both directions ±ΔS. For this, one or more switching valves are opened which are closed again after completion of the volume shift. The position of the piston 3, e.g. at the start, middle or end of the stroke, is irrelevant for ΔP volume control for the regulation. Here, during the pressure change, a temporal control may be used in order to implement transition functions towards the end of the pressure change, e.g. to reduce the pressure fluctuations and the associated noise.

A high dynamic is important if two or more wheels require a pressure change simultaneously. For this, the invention proposes that to relieve the load on the motor dynamics, one or two additional outlet valves AV are used. For volume control, in particular also the pressure level in the pressure-generating unit DE and in the wheels is important. It is favourable here that the pressure level on pressure change corresponds to the starting pressure of the wheel to be regulated. This achieves a rapid and low-noise pressure regulation. The temporal developments are illustrated in FIGS. 3, 3 a, 3 b and 4, 4 a, 4 b.

Suitable pressure-generating units DE are all pumps with single piston, stepped piston, double-stroke piston and also e.g. gear pumps which allow precise volume control.

In FIG. 1 a , for the above-mentioned functions, the pressure generation takes place in one circuit directly via an isolating valve TV1 in BK1 (Var2) or alternatively via a blow hole SL on the front of the SK piston (Var1). The pressure is supplied to brake circuit BK2 via an isolating valve TV2. For infeed via the blow hole SL on the front of the floating piston SK, optionally the isolating valve TV1 may be omitted since, on system failure, the pressure-generating unit DE is isolated from the master brake cylinder effect because piston SK moves and shuts off the pressure supply DE. Alternatively, as drawn in dotted lines, the pressure-generating unit DE may be connected directly to BK1 via TV1 (Var2). Since the SK piston in Var1 is only moved in fall-back level, a special diagnosis circuit is required in which the floating piston SK is moved and checked for tightness.

FIG. 1 b describes the known pressure control based on a simplified pressure-volume curve which forms the basis of the MUX regulation. Depending on the required pressure difference Δp, from the curve a volume change ΔV is read which is implemented as a travel change Δs of the piston 3 by shifting the plunger of the pressure-generating unit DE. This applies for both pressure build-up and pressure reduction.

FIG. 1 c shows a fundamental possibility for pressure regulation in the basic embodiment of FIG. 1 . The system itself has the following degrees of freedom in pressure regulation:

-   -   pressure build-up and pressure reduction in all brake circuits         BKI and BKII mainly with multiplex regulation (pressure         regulation with pressure-volume control) in all wheel brake         cylinders simultaneously or sequentially;     -   multiplex regulation in pressure build-up and pressure reduction         in brake circuit I via the opened isolating valve TV1 and         simultaneous pressure reduction in brake circuit II via the         outlet valve AV3;     -   multiplex regulation via SV1, SV2, SV4 in pressure build-up and         pressure reduction in brake circuit I and II for wheel brakes         RB1, RB2 and RB4, and simultaneous pressure reduction in wheel         brake RB3 via opened outlet valve AV3 with closed switching         valve SV3.

The pressure reduction p_(ab) via switching valves SV1 and SV2 in BK I takes place mainly via pressure-volume control, sequentially or simultaneously. For this, the respective switching valve SVi is always opened. For simultaneous pressure reduction p_(ab) at different starting pressures, optionally by deviation from the MUX regulation, switching valves SV1 and SV2 may be opened with a time offset, and pressure reduction p_(ab) controlled via a switching valve SV2. The isolating valve TV1 is always opened on pressure reduction. In this exemplary embodiment, the wheel brake RB1 has a higher pressure, therefore the associated switching valve SV1 is opened before the switching valve SV2.

On the basis of knowledge of the pressure difference—the pressures in wheel brakes RB1 and RB2 and the pressure in the pressure-generating unit DE are known—the time control may be dimensioned precisely. SV2 is opened when the pressure in the pressure-generating unit DE is approximately reached. Further pressure reduction then takes place simultaneously in both wheel brake cylinders RB1 and RB2 by control via piston 3 when switching valves SV1, SV2 and TV1 are open. When the target pressure of a wheel is reached, the corresponding switching valve SV1 or SV2 is closed. If further pressure reduction is desired in one wheel, the further pressure reduction can take place in the respective wheel brake.

As already described, to simplify the system, preferably the PWM control is omitted, in particular also for noise reduction.

Exemplary temporal curves of the pressure reduction are described in FIGS. 4 a to 4 c.

FIG. 1 d shows a conventional valve circuit for ABS with four inlet valves EV and four outlet valves AV. If this is also used with fewer AV for MUX, e.g. with differential pressure in MUX, as well as this pressure the fault case must be taken into account in which, on an asymmetric road surface, the pressure-generating unit and also the valve actuation fail suddenly e.g. due to the ECU, and at the same time the pressure-generating unit has a low pressure level. In this case for example, EV1 has 130 bar and EV2 0 bar. On failure of the pressure-generating unit, this means that for EV1 the return spring of the valve armature must open around 130 bar. To enable this, the magnet circuit of the valve must be sufficiently large, whereby the valve becomes costly. Alternatively, a pressure-relieved valve may be used, but its costs are also high.

In dimensioning of the valve seat, it must also be taken into account that this should be as large as possible in order to generate a small backup pressure if the brake pressure is to be built up rapidly by the pressure-generating unit. The backup pressure is introduced directly into the motor torque or power.

FIG. 1 e shows a modified throughflow of the switching valves SV. The hydraulic medium flows from the brake circuit or pressure-generating unit through the armature chamber to the valve seat and onto the wheel cylinder. If the above fault case occurs, the wheel pressure opens the switching valve. The magnetic force must however also close around 130 bar, which takes place however with a small armature air gap in the valve end position. The return spring of the switching valve SV need therefore only be slightly strengthened so that the switching valve does not “snatch” at correspondingly high volume flow. Since conventional inlet valves must close at around 220 bar—in FIG. 1 e, 130 bar—, with the same magnet dimensioning the valve seat area may be increased which means a smaller backup pressure or flow resistance and is advantageous for MUX mode. The valve circuit depicted in FIG. 1 e is therefore advantageous for the brake system according to the invention.

FIG. 1 f shows a possible embodiment of the inlet valve EV according to the invention and the connection to the brake circuit BK and the pressure supply DV and wheel brakes RBi.

The inlet valve EV has a magnet armature MA, a magnetic base body MGK and an exciter coil ES. When the magnetic valve EV is powered, the magnet force MK shifts the armature MA from position S_(A0) to position S_(A2) by the differential travel SA. The magnet armature MA moves a ram MStö by the same travel, so that the ram MStö comes to rest on the valve seat VS and closes the outlet Ea of the magnetic valve. The armature MA at this point still has a residual air gap S₀ from the magnetic base body MKG, which is provided so that the armature MA does not stick to the magnetic housing MGK when the power to the exciter coil ES of the valve EV is switched off, due to re-magnetisation losses of the iron circuit. When the valve current is switched off, the return spring RF moves the armature MA back to the starting position. The magnet force F_(M) rises nonlinearly with a smaller air gap, e.g. with increasing travel. The return spring F_(RF) is dimensioned such that the magnetic force F_(M) in the starting position S_(A0) is greater than the spring force, so that a secure closure of the valve is guaranteed. The spring force increases with the increasing travel SA and in the end position S_(A2) is however smaller than the magnet force F_(M). Preferably, a linear spring is used, so that the magnet force F_(M) in the end position for a given current is significantly higher than the return force, so that the valve can be retained with low current, or secure closure is guaranteed even at high differential pressures between the wheel brake and the pressure supply. This retention is also ensured at high differential pressures since the magnet force increases strongly nonlinearly at the closed valve position. The return spring must however also be dimensioned such that the function as an unpowered open valve can be ensured and the valve always opens safely.

The outlet E_(a) of the valve is connected to the wheel brakes RBi (RB1-RB4), the inlet E_(i) to one brake circuit BKi or to the pressure supply unit DV (20). With such connections, the inlet valve EV can be opened both by the return spring RF and by the pressure in the wheel brake, which is very important in particular in the event of a fault or malfunction in the brake system (e.g. loss of voltage to the valve). Also, even at high pressures in the brake circuit and small pressures in the wheel brake, only the pressure difference between inlet Ei and outlet Ea acts on the ram MStö. This differential pressure at the valve is relatively low in pressure build-up, but must however be taken into account in the spring design RF so that the pressure difference does not lead to the valve being pushed back on pressure build-up when the volume of the pressure supply DV is delivered to the wheel brake. Valves with large opening cross-section ÖQ or low flow losses reduce this effect.

In particular on pressure build-up with pressure-volume control or time control with low differential pressure between the pre-pressure and actual pressure in the wheel brake, the valves described above with large opening cross-section may be used since the regulation accuracy is very high. This in turn has advantages in that only low flow losses occur, in particular with rapid pressure build-up (TTL), and the drive motor requires only a low power for rapid pressure build-up in a very short time (TTL=150 ms).

Also, because of the low flow losses of the advantageously configured inlet valves, a pressure reduction can take place quickly via the inlet valves. Precise pressure reduction via the inlet valves EV can take place with corresponding control of the piston movement of the pressure supply unit 20. Optionally, it is also possible to implement the known MUX process with the valve circuit described above, or with pressure reduction control via outlet valves AV in one brake circuit, in particular for consumers with low volume balance, e.g. the wheel brakes on the rear axle. In other words, a combination is also possible which uses the MUX process in connection with the new valve circuit only in two wheel brakes (e.g. front axle), and the pressure reduction takes place conventionally on two further wheel brakes. This would mean that two wheel brakes/actuators are provided with inlet and outlet valves (EV+AV) and two wheel brakes/actuators only with inlet or switching valves EV. In this case, only the wheel brakes of the front axle are equipped with the new valve circuit according to the invention as shown in FIGS. 1 a and 1 b , and a standard circuit/standard valves are used on the rear axle.

FIG. 2 a shows the pressure-volume curve of the wheel/wheel cylinder with connecting lines as far as the switching valve SV and pressure sensor. Two curves are shown. Curve P_(aufa) corresponds to a so-called stiff curve, the other curve P_(auf) requires substantially more volume. This may in extreme cases cause vapour bubbles due to e.g. play or poor purging.

This means that the values for V_(a) e.g. for ΔP=P₁−P₂ are equal to V₁−V₂=ΔV_(a)=ΔS_(a) and at V_(auf)=Δ_(P) equal to V_(1a)−V_(2a)=ΔV=ΔS. This curve for p_(auf) and p_(ab) is stored e.g. for the first time on line-end tests in the memory of the control device both for the individual wheel brakes and for the brake circuits for both p_(auf) and p_(ab). On each braking, the curve is measured by comparison of pressure P with the volume V_((Δs)). If a great deviation occurs, with a stationary vehicle, the curves can be recorded or adapted as in the above-mentioned test. It is also significant that the values can fluctuate between P_(auf) and P_(ab). It is normal that due to play, V₀ is greater on pressure build-up P_(auf) but not on pressure reduction P_(ab). When the play has been eliminated, the curves are almost equal.

With poor purging or vapour bubbles, the curves behave similarly but with greater volume for the corresponding pressure value.

For regulation, the p-V curves are used for pressure build-up p_(auf) and pressure reduction p_(ab).

FIG. 2 b describes the simplified relevant pressure-volume curves for pressure regulation without hysteresis in a closed brake circuit or shifting after pressure reduction with opened outlet valve AV. Starting from a pressure p1, by defining a nominal differential pressure Δp, the necessary volume shift ΔV or travel change Δs of the piston can be read from the curve. These differ and are dependent on whether the pressure is changed in one or more brake circuits. The piston is then moved accordingly. If the pressure is reduced via one or more outlet valves, there is a volume loss in the pressure-generating unit. For further pressure reduction or pressure build-up in the closed brake circuit, the travel allocation of the pressure-volume curve is determined by detecting the pressure. This is required in regulation for monitoring the volume balance, since the working chamber of the pressure-generating unit only has a limited volume and thus towards the end of the piston stroke, the piston would travel to the stop. If the piston of the pressure-generating unit travels close to the stop after a pressure change and a further pressure rise is impending, the piston is retracted briefly with closed switching valves SV in order to draw in volume from the storage container. In the design with double-stroke piston (FIG. 5-6 ), this is retracted or switched to return stroke mode.

FIG. 3 shows a time development of the MUX regulation as known from WO 2006/111393 A1 or WO 2010/091883 A1. This system is known as the 4-channel MUX in which, except in the critical case of simultaneous pressure reduction (simultaneous p_(ab)), the pressure reduction p_(ab) is processed serially per wheel channel (cylinder). In the worst case scenario, this leads to a great delay time which causes large speed differences or even slip because of the individual response times of the valve, motor and the time for the respective pressure reduction p_(ab). This both reduces the stability of the braking and disadvantageously extends the braking distance. Optimisation is performed on the response time of the switching valves, motor and pressure reduction gradients. Costs however limit optimisation. The case of simultaneous pressure reduction p_(ab) for all channels however occurs rarely in practice.

A further restriction exists in the regulation concept known from WO 2006/111393 A1 or WO 2010/091883 A1 in the necessary priority for the pressure reduction p_(ab). If a pressure reduction is required, no pressure build-up p_(auf) can take place. Since usually the time for pressure build-up p_(auf) in the regulation cycle is around 200 ms, and two or three small p_(auf) take place per control cycle each with approximately 10 ms delay time, this was not considered critical but is noted as a minor defect of the 4-channel MUX.

The brake system according to the invention with its regulation concept offers the following improvements:

-   -   introduction of an additional outlet valve on the front axle;     -   various control methods and strategies for control and         regulation of the wheel brakes, e.g. VA, corner braking;     -   possibility of pressure build-up p_(auf) with simultaneous         pressure reduction p_(ab) (described in FIG. 5 b and FIG. 5 c ).

FIG. 3 shows the pressure curve in the individual wheel brakes over time for conventional MUX mode in which the pressure in the wheel brakes is reduced temporally successively. V1 and V2 are the front wheel brakes, H1 and H2 the rear wheel brakes. At X, the signal is given for simultaneous pressure reduction p_(ab). The response time t_(v)SV of the switching valve SV is around 5 ms. The response time t_(v)M of the motor is around 10 ms. It is taken into account here that the pistons of the pressure-generating unit must first be positioned at (1) for the different pressure levels of the individual wheels before the pressure change. Then the pressure reduction p_(ab) takes place, assuming t_(ab) of around 10 ms, wherein during this time the pressure is reduced by Δp equal to around 20 bar.

The response times for the switching valves and motor assumed in FIG. 3 are also assumed, for an objective comparison, for the depiction of the temporal pressure development in FIG. 3 a which corresponds to the pressure development according to the invention.

With conventional ABS systems with 4 inlet and 4 outlet valves, the ABS regulator always determines a Δp and then determines the time for which the outlet valve must be opened in order for the required pressure reduction to take place in the wheel brake. This time control is known to be subject to tolerances, which limits the precision of the pressure regulation. Also, on closure of the outlet valve AV, pressure fluctuations always occur which cause disadvantageous noise.

The ABS regulator here determines the necessary pressure difference Δp substantially from the wheel angular acceleration and partially from the wheel slip, with correction factors for a) wheel inertia moment, b) gear stage and c) fading detection.

In contrast to time control, in conventional MUX as shown in FIGS. 1 and 1 a, a volume control of the pressure supply is used in which Δv=Δp, wherein this takes place with evaluation of the pressure-volume curve of the wheel. Thus the precision of the pressure regulation is much greater, and the temporal pressure development can be influenced towards the end of the pressure reduction so that only slight pressure fluctuations occur.

At V1 in FIG. 3 , after (X), the response time of t_(v)M and t_(v) of SV1 acts. After opening of the switching valve SV1, the motor M is able to reduce the pressure over the time t_(ab). Then SV1 is closed again at (2). First however, the motor has already reached the required pressure level via the described volume control of the pressure supply.

Then the motor already begins the pressure reduction p_(ab) of the front wheel V2, which takes place after opening of SV2 of V2. Thus the sequence V1-H2 has a total delay time of 60 ms with the above assumptions. This corresponds approximately to a regulation deviation Δv of around 15 km/h.

FIG. 3 a shows the temporal pressure curve for the regulation concept according to the invention. The pressure reduction p_(ab) at the front wheel brake V1 at time (1) corresponds to (1) in FIG. 3 . At the front wheel brake V2, the additional outlet valve AV is used for pressure reduction p_(ab). The pressure reduction p_(ab) takes place almost without delay at (11) via a time control Δt described above, at which at (12) after closure of SV2 the pressure fluctuation occurs. Thus regulation of the front wheels V1 and V2—which at high μ make a considerably greater contribution to the brake force effect than the rear wheels—is almost not delayed. During pressure reduction p_(ab) of V1, the motor at (13) is already prepared for pressure change at the rear wheel H1. This take place at (14) to (15) by volume control by means of the pressure-generating unit DE. Since the rear wheels H1, H2 often have the same higher pressure level than V1 or V2, a simultaneous pressure reduction p_(ab) can take place by volume control. Alternatively, at H2 at (16) a time-controlled pressure reduction p_(ab) into the brake circuit can take place. One condition for this is that the MUX pressure level in the wheel brake of the rear wheel H2 is lower than at the rear wheel H1, at which the pressure reduction takes place by means of volume control. It is also possible that the pressure reduction in the rear wheel brake H2 takes place by time-controlled opening of the associated outlet valve.

Variants of the front axle and rear axle regulation are shown in detail and described in FIGS. 4, 4 a, 4 b. A comparison of the delay times tv in relation to the conventional MUX methods in FIG. 3 show a significant improvement at (10) with 60 ms and (17) with 25 ms. This is possible by the use of the time-controlled pressure reduction p_(ab) at V2 and H2 by means of outlet valve or into the brake circuit, partially simultaneous p_(ab) at H1 and H2, and priority control at H1 and (13).

FIGS. 4-4 b show the pressure development with different actual pressure levels and variants of the front and rear wheels H1, H2, V1 and V2 with the regulation concept according to the invention.

FIG. 4 shows the temporal pressure development V1 to H2 in different phases. Phase 0-X shows a pressure development in which, because of the different pressure levels in the wheel brakes, no simultaneous pressure build-up p_(auf) and pressure reduction p_(ab) can take place, which is most often the case. Consequently, here also full multiplex mode is active i.e. precise Δp regulation via volume control both for pressure build-up p_(auf) and for pressure reduction p_(ab). At pressure build-up p_(auf), sometimes e.g. at 20 with simultaneous pressure build-up requirement, a temporally offset pressure build-up p_(auf) takes place at H1 and H2. However also a partially simultaneous pressure build-up p_(auf) may take place. Also, a partially simultaneous pressure reduction is possible as shown in FIG. 4 b.

At X in FIG. 4 , the signal p_(ab) for simultaneous pressure reduction in all wheel brakes is given, which is implemented without time delay. These two variants A and B are shown in more detail in FIG. 4 a and FIG. 4 b.

FIG. 4 a shows the variant A, again starting at point X, with p_(ab) at V1 and V2 as described in FIG. 3 a . At the front wheel V1, the pressure reduction p_(ab) takes place by means of volume control by the pressure-generating unit DE. For the rear wheel H2, at 21 a controlled p_(ab) takes place over time dt=f(dp). This pressure reduction takes place when a sufficient differential pressure Δp between H2-V1 is present. The pressure levels of all wheels are known in the regulation concept according to the invention or MUX method, so that a relatively precise pressure reduction p_(ab) in the rear wheel H2 is achieved by the time control or opening of the switching valve SVH2. The necessary opening time Δt may, because of the change in pressure level, be adapted flexibly by M1 (MUX). The pressure reduction p_(ab) of the rear wheel H1 also takes place via volume control, starting from the preparation at 13, and then by opening of the associated switching valve at 22.

As a result, there is a relatively small tvmax as described in FIG. 3 a . At 11, the time-controlled pressure reduction p_(ab) for the front wheel V2 takes place. Here again, the pressure difference from the storage container is known and hence a precise pressure control is possible by the time-controlled opening of the outlet valve.

FIG. 4 b shows variant B for partially simultaneous pressure reduction at the rear wheels H1 and H2, starting from a relatively small pressure difference between the rear wheels H1 and H2. Here, after preparation at 13, at 23 the pressure reduction p_(ab) takes place with MUX, i.e. volume control by means of the pressure-generating unit. At 24, the rear wheel H2 is switched to pressure reduction p_(ab) by opening of the switching valve SVH2 assigned to the rear wheel H2. At 25, the controlled pressure reduction p_(ab) for H2 is achieved, so the switching valve SVH2 is closed. At 26, via the volume control, the Δp for the rear wheel H1 is reached, so at 26 the switching valve SVH1 is closed.

Both methods allow a short delay time. In some cases, the controlled pressure reduction p_(ab) causes the pressure fluctuations, which however only occur in extreme cases with simultaneous pressure reduction p_(ab).

To summarise and in addition, the following features apply:

-   -   the pressure of each wheel at the start and end of the pressure         reduction p_(ab) (FIG. 4, 4 a) is stored in the memory; the two         values are used as reference for the subsequent pressure changes         of the wheel or following wheels;     -   the pressure of the last pressure build-up p_(auf) (FIG. 4 ) is         stored in the memory and thus forms the basis for setting the         pressure of the pressure-generating unit DE in preparation for         the following pressure reduction p_(ab);     -   the outlet valves AV are time-controlled, wherein for this the         pressure difference from the storage container is taken into         account; one wheel of the front axle is connected to the brake         circuit only via switching valve SV, wherein a switching valve         SV towards the brake circuit and an outlet valve AV towards the         storage container are assigned to the second wheel of the front         axle, so that switching valves SV of the rear axle are         time-controlled to p_(ab), while MUX controls the wheels of the         front axle with low pressure level, in which the time control         t_(ab) of the switching valves SV of the rear axle HA evaluates         the differential pressure Δp;     -   as well as the differential pressure, for the time control         (valves are opened for a predefined time), the Δp of the         corresponding outlet valve AV is evaluated from the         pressure-volume curve. Priority control of the MUX with         orientation towards driving stability with braking distance,         e.g. wheel of the front axle VA has priority and also, at         positive μ jump with highest negative p_(ab) or positive         acceleration p_(auf), since here the Δp to be regulated is         greatest;     -   the pressure change by volume control by means of the         pressure-generating unit, and the time-controlled opening of         outlet valve or switching valve used in parallel, forms the         combined MUX regulator; in time control, the corresponding         volume must be taken into account in the volume delivery,         corresponding to the pressure change Δp determined by the         regulator.

FIG. 5 describes a further embodiment of the pressure supply unit DE according to the invention with pressure regulation with master brake cylinder, master brake cylinder valves HZV, pressure supply with double-stroke piston, switching and outlet valves.

The master brake cylinder HZE is connected to brake circuits BKI and BKII. For the separation logic, the same applies as in FIG. 1 a . The advance stroke chamber 4 of the double-stroke piston 3 is connected to brake circuit BI via the isolating valve TV1, and to BK II via the isolating valve TV2. The return stroke chamber 4 a is connected via the isolating valve TV2 b to BK II and via HZE to BK I. The transmission preferably takes place by the floating piston SK. The advance stroke chamber 4 and return stroke chamber 4 a can be connected together hydraulically via a switching valve ShV. This switching valve SvH allows a short-circuit between the two chambers, and is used in particular on advance stroke (towards the left) to reduce the hydraulically active area of the piston 3. The return stroke chamber 4 a of DHK 3 is connected via the switching valve PD1 to the storage container 10. The two chambers 4 and 4 a are also each connected via a check valve to the storage container 10. This system configuration offers the following degrees of freedom:

-   -   pressure build-up and pressure reduction in all brake circuits         with multiplex regulation (pressure regulation with         pressure-volume control) in all wheel brake cylinders         simultaneously or sequentially via isolating valves TV1, TV2 and         PD1, and the switching valves SV1-SV4 of the wheel brakes         RB1-RB4;     -   multiplex regulation in pressure build-up and pressure reduction         in brake circuit I and pressure reduction in brake circuit II         via outlet valves AV3, ZAV;     -   multiplex regulation with simultaneous pressure reduction in         brake circuit BK I and pressure build-up in BK II via         double-stroke piston control;     -   multiplex regulation with simultaneous pressure reduction in         brake circuit BK II and pressure build-up in BK I via         double-stroke piston control;     -   pressure reduction RB3 at any time via AV3 in multiplex         regulation.

FIG. 5 a shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other:

-   -   controlled pressure reduction in RB1 via SV1, TV1 by         pressure-volume control by means of return stroke of the         double-stroke piston 3 with open PD3 valve, or alternatively         pressure control of pressure reduction via pressure estimation         based on phase current measurement in brake circuit I;     -   controlled pressure reduction in RB2 via SV2, TV1 via         pressure-volume control by means of return stroke of         double-stroke piston 3 with open PD3 valve, or alternatively         pressure control of pressure reduction via pressure estimation         based on phase current measurement in brake circuit I;     -   pressure reduction in RB3 via AV3 with time control of outlet         valve AV3; pressure reduction in RB4 by means of double-stroke         piston 3 with time control of one of the switching valves SV4 or         PD1, wherein the other valve must also be opened at this time,         or alternatively pressure control of pressure reduction via         pressure estimation based on pressure measurement in brake         circuit II.

For simultaneous pressure reduction p_(ab) at different starting pressures, optionally a deviation may occur from the MUX regulation in that the switching valves SV1 and SV2 are opened with time offset. The isolating valve TV1 is here opened continuously on pressure reduction. Since a higher pressure prevails in RB1, the switching valve SV1 is opened before switching valve SV2. On the basis of knowledge of the pressure difference (wheel pressure RB1 and RB2 and pressure in the advance stroke chamber of the pressure supply unit), the time control can be dimensioned precisely. If the pressure in the advance stroke chamber of the pressure supply unit DE is not determined precisely, because at the same time a pressure reduction via ZAV is taking place in wheel brake RB4, and TV2 is closed, the pressure in the advance stroke chamber may be used via pressure estimation p/i from the torque of the electric motor. The switching valve SV2 is open when the pressure of the pressure-generating unit DE is approximately reached. The further pressure reduction then takes place simultaneously in both wheel brake cylinders by control via piston 3 with open SV1, SV2 and TV1. When the target pressure of a respective wheel is reached, the corresponding valve SV1 or SV2 is closed. If further pressure reduction is required in a wheel, further pressure reduction can take place only in one wheel brake.

In parallel to the pressure reduction control in MUX mode, in BK II the pressure can be reduced by time control of AV3. This can be determined temporally freely because closure of SV3 does not influence the other wheel brake cylinders. Also, the temporal activation of the pressure reduction in wheel brake RB4 can be selected freely on pressure reduction of BK 1 in MUX mode.

FIG. 5 b shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other:

-   -   controlled pressure reduction in RB1 via switching valves SV1         and TV1 via pressure-volume control by means of return stroke of         double-stroke piston 3 with closed PD1 valve;     -   controlled pressure reduction in RB2 via switching valves SV2         and TV1 via pressure-volume control by means of return stroke of         double-stroke piston 3 with closed PD1 valve;     -   pressure reduction in RB3 via outlet valve AV3 with time control         Δt (opening of AV3 valve for period at);     -   pressure build-up in RB4 via isolating valve TV2 b (ZAV) with         pressure-volume control by means of return stroke of         double-stroke piston with closed PD1 valve.

For simultaneous pressure reduction and pressure build-up in wheel brake RB4, the pressure build-up dynamic is determined by the pressure reduction dynamic and the effective piston area and hydraulic differential pressures. This must be taken into account in the regulation. When the target pressure is reached in wheel brake RB4, the switching valve SV4 is closed. If the pressure in BKI is to be reduced further, PD1 is opened for further pressure reduction in brake circuit I.

FIG. 5 c shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other:

-   -   controlled pressure reduction in RB1 via switching valves SV1         and TV1 via pressure-volume control by means of advance stroke         of double-stroke piston 3 with open PD1 valve;     -   controlled pressure reduction in RB2 via switching valves SV2         and TV1 via pressure-volume control by means of advance stroke         of double-stroke piston 3 with open PD1 valve;     -   pressure reduction in RB3 via outlet valve AV3 with time control         of outlet valve AV3;     -   pressure reduction in RB4 via double-stroke piston 3 with time         control of switching valve SV4 or PD1 valve.

For the many functions of pressure reduction p_(ab) in one brake circuit and pressure build-up p_(auf) in the other brake circuit, it is possible for the floating piston SK of the master brake cylinder HZE to move. To prevent this, a blocking element SE may be arranged in BK1 or BK2 which acts directly on the SK as mechanical blocking. The blocking element may also be part of the HZV.

With this pressure regulation system, the functions described in 5 b and 5 c of p_(auf) in one brake circuit and p_(ab) in the other can be implemented independently of the pressure level of the brake circuits.

FIG. 5 d shows as an example the pressure reduction in brake circuit I and brake circuit II which is implemented on pressure reduction from high pressures. With isolating valves TV1 and TV2 open, the following is carried out:

-   -   controlled pressure build-up in RB1 by time control of valves         SV1 and PD3 via pressure control of the pressure reduction by         pressure estimation based on phase current measurement in brake         circuit I;     -   controlled pressure build-up in RB2 by time control of valves         SV2 and PD3 via pressure control of the pressure reduction by         pressure estimation based on phase current measurement in brake         circuit I;     -   controlled pressure build-up in RB3 by time control of valves         SV3 and PD3 via pressure control of the pressure reduction by         pressure measurement based on the pressure sensor in brake         circuit II;     -   controlled pressure build-up in RB4 by time control of valves         SV4 and PD3 via pressure control of the pressure reduction by         pressure measurement based on the pressure sensor in brake         circuit II.

For pressure reduction for individual wheels, in the same way as shown in in FIG. 5 a , the switching valves SV1-SV4 may be switched with a temporal offset.

One possibility (not shown) is that of pressure reduction via PD1 valve which is similar to the process for PD3 valve. The pressure reduction may take place for all brake circuits via the PD1 valve. The pressure reduction may also take place via PD3 and PD1 valve. This is similar to FIG. 5 a with the difference that the pressure of all wheel brakes is reduced via the pressure supply unit, and hence the advantages of pressure reduction in the closed brake circuit are obtained, which has safety advantages in particular after completion of a braking process (e.g. after ABS operation).

FIG. 6 describes a system with double-stroke piston in advantageous dual circuit design. The structure of THZ, DE and the valve circuits for pressure control ABS with MUX and AV is identical to FIG. 5 c.

in contrast to FIG. 5 c , the pressure supply in the advance stroke acts on brake circuit II and the back of the floating piston SK. This transmits the volume and pressure to brake circuit I. If the double-stroke piston has travelled close to the end position, it is reversed and operated in return stroke and acts on BK I. Then via the return stroke, pressure acts on the front side of the floating piston SK. This transmits the pressure again to brake circuit BK II. The SK piston is always active with its seals, as in the current THZ.

The double-stroke piston 3 also has a bypass valve ShV, which is switched substantially under three conditions:

-   -   a) at high pressure, to reduce the piston force, the volume of         the advance stroke is also conducted to the back of the         double-stroke piston 3 to balance the pressure;     -   b) in ABS regulation and also MUX regulation, the double-stroke         piston 3 is switched in a single circuit via the ShV valve;     -   c) pressure reduction p_(ab) from high pressure level takes         place simultaneously in both brake circuits BK I and BK II.

This valve circuit has the consequence for the floating piston position that the return spring 1 moves the floating piston SK to the right stop or locks it in the middle position. The pressure sensor 9 measures the pressure in BK II and, with a “single circuit” arrangement for the regulation and control functions, can evaluate the pressure in both brake circuits.

For special functions with pressure build-up p_(auf) in BK I and pressure reduction p_(ab) in BK II and vice versa, it is advantageous to place in the connection to THZ in BK II, or a blocking element SE in brake circuit BK 1 which prevents the movement of the floating piston SK. The blocking valve SE may also be part of the HVZ.

This system contains the additional potential for reducing the pressure in BK II via the DHK piston 3, and separately from BK I via the valves TV2 b and PD1.

This solution has advantages in use for different pressure level activation on the two axles in recuperation. For this, the blocking element SE must then be used at SK or in BK I.

The functions described in FIGS. 5-6 and the additional overlaid time control via outlet valve(s) give the MUX system according to the invention a very good performance with high regulation dynamic and precision, with significantly lower cost than individual wheel regulation with inlet and outlet valves. 

What is claimed is:
 1. A brake system for motor vehicles, the brake system including: an actuation device, a travel simulator configured to generate a feedback force on the actuation device, a first piston-cylinder unit, having at least one piston that separates two working chambers from each other, wherein each working chamber is connected via at least one hydraulic connecting line to at least one wheel brake of a brake circuit, wherein at least one wheel brake is assigned to each brake circuit and each wheel brake is enabled to be connected to an associated one of the hydraulic connecting lines via an associated controllable switching valve, a control device, at least one pressure supply unit configured to be driven by an electric motor and having at least one working chamber, wherein the at least one pressure supply unit is configured to build up and/or reduce brake pressure in one or more of the wheel brakes simultaneously or successively, and at least one outlet valve, wherein a respective one of the at least one outlet valve is associated only with a single wheel brake or is associated with one wheel brake of a respective brake circuit, wherein the outlet valve is arranged in a hydraulic connection between the wheel brake with which it is associated and a pressure medium storage container, wherein no further valve is arranged between the respective one of the at least one outlet valve and the pressure medium storage container, and wherein the pressure supply unit comprises one pressure chamber, wherein the one pressure chamber is connected via a hydraulic connection to a working chamber of the first piston-cylinder unit, wherein the hydraulic connection is enabled to be shut off by movement of the piston of the first piston-cylinder unit into a shut-off position.
 2. The brake system according to claim 1, wherein a respective one of the at least one outlet valve is associated with a front wheel brake.
 3. The brake system according to claim 1, wherein an outlet valve associated with a wheel brake of one of the brake circuits serves for pressure reduction in another one of the wheel brakes of the one of the brake circuits, wherein for common pressure reduction in one of the brake circuits, the control device is configured to open both the switching valves and the respective outlet valve associated with the one of the brake circuits.
 4. The brake system according to claim 3, wherein during opening of the switching valves associated with the one of the brake circuits, the control device is configured to separate the one of the brake circuits from the pressure supply unit by closing an associated isolating valve or by locking the piston of the first piston-cylinder unit.
 5. The brake system according to claim 1, wherein, for simultaneous or temporally offset pressure reduction and/or pressure build-up using the pressure supply unit, the control unit is configured to set a pressure in its at least one pressure chamber and to open and/or close the switching valves simultaneously and/or with temporal offset in order to set the nominal pressure required in the respective wheel brakes, wherein, by means of the at least one outlet valve, pressure is reduced in the wheel brake or brakes associated with the at least one outlet valve, independently of the pressure supply unit.
 6. The brake system according to claim 1, wherein for simultaneous and/or temporally offset pressure reduction and/or pressure build-up, the control device is configured to actuate the pressure supply unit, taking into account wheel brake pressures calculated or prevailing in the wheel brakes, wherein the pressure supply unit is configured to generate a nominal pressure to be set for a respective wheel brake with a respective open switching valve, and after reaching the nominal pressure, the control device is configured to close the respective switching valve to maintain the nominal pressure in the respective wheel brake, wherein the at least one outlet valve is configured to enable pressure to be reduced in a wheel brake associated with a respective one of the at least one outlet valve independently of the pressure-generating unit.
 7. The brake system according to claim 1, wherein at least one pressure sensor serves to determine the pressure in at least one brake circuit.
 8. The brake system according to claim 1, wherein, for pressure build-up and/or pressure reduction in at least one wheel brake, the control device is configured to evaluate a pressure-volume curve of each wheel brake and from a respective pressure rise or fall to be generated, to calculate piston travel of a piston of the pressure supply unit required for the respective pressure rise or fall to be generated, and to correspondingly actuate required valves, and to move the piston of the pressure supply unit by an amount of the piston travel, using the electric motor.
 9. The brake system according to claim 1, wherein the control device is configured to determine a pressure in one brake circuit by means of a pressure sensor, and to determine a pressure in another brake circuit via a phase current of the electric motor of the power supply unit, and to take into account determined or calculated brake circuit pressure in controlling pressure build-up or pressure reduction in the wheel brakes.
 10. The brake system according to claim 1, wherein the switching valve is a digital valve that is open in an unpowered state.
 11. The brake system according to claim 1, wherein, on pressure build-up and/or on pressure reduction, the control device is configured to take into account an absolute position of a piston of the pressure supply unit and to determine a necessary movement travel of the piston of the pressure supply unit, depending on actual pressures, nominal pressures and pressure-volume curves of the respective wheel brakes involved in the pressure build-up or reduction.
 12. The brake system according to claim 11, wherein, in normal operation with normal brake force amplification, recuperation, and anti-lock braking system (ABS) at low μ, pressure is built up and reduced simultaneously or sequentially in all wheel brakes via the movement travel of the piston of the pressure supply unit, taking into account the respective pressure-volume curves, and wherein, in operating situations with ABS at high μ, μ split, electronic stability program (ESP) and anti-slip regulation (ASR), pressure in at least one wheel brake is reduced via at least one associated outlet valve and/or via one or more outlet valves of the pressure supply unit at the same time.
 13. The brake system according to claim 1, wherein, by means of the actuation device, upon failure of the pressure supply unit or upon occurrence of another fault the at least one piston of the first piston-cylinder device is enabled to be set to build up pressure in at least one wheel brake.
 14. The brake system according to claim 1, wherein for the building up and/or reducing brake pressure performed by the pressure supply, a pressure measured by a pressure sensor is considered.
 15. The brake system according to claim 1, wherein for the building up and/or reducing brake pressure, a pressure is evaluated by a pressure estimation based on phase current of the motor of the pressure supply.
 16. The brake system according to claim 1, wherein pressure is built up and reduced simultaneously or sequentially in all wheel brakes via the movement travel of the piston of the pressure supply unit, taking into account a respective pressure-volume curve of each wheel brake.
 17. The brake system according to claim 1, wherein the pressure supply has at least one of a pump with a single piston and only one pressure chamber, or a pump with a stepped piston or double-stroke piston and two working chambers.
 18. A brake system for motor vehicles, the brake system including: an actuation device, a travel simulator configured to generate a feedback force on the actuation device, a first piston-cylinder unit, having at least one piston that separates two working chambers from each other, wherein each working chamber is connected via at least one hydraulic connecting line to at least one wheel brake of a brake circuit, wherein at least one wheel brake is assigned to each brake circuit and each wheel brake is enabled to be connected to an associated one of the hydraulic connecting lines via an associated controllable switching valve, a control device, at least one pressure supply unit configured to be driven by an electric motor and having at least one working chamber, wherein the at least one pressure supply unit is configured to build up and/or reduce brake pressure in one or more of the wheel brakes simultaneously or successively, and at least one outlet valve, wherein a respective one of the at least one outlet valve is associated only with a single wheel brake or is associated with one wheel brake of a respective brake circuit, wherein the outlet valve is arranged in a hydraulic connection between the wheel brake with which it is associated and a pressure medium storage container, wherein no further valve is arranged between the respective one of the at least one outlet valve and the pressure medium storage container, and wherein the control device has a memory configured to store actual wheel brake pressures set in each wheel brake and/or probable wheel brake pressures of each wheel brake continuously calculated and updated with measured values in a control model, wherein, for simultaneous pressure reduction in at least two wheel brakes of one brake circuit, to different nominal pressures in the at least two wheel brakes, the control unit is configured to evaluate a pressure-volume curve of each of the at least two wheel brakes of the one brake circuit and, using a lowest nominal pressure to be generated, is configured to calculate a piston travel of a piston of the pressure supply unit required and to move the piston of the pressure supply unit by the piston travel by means of the electric motor of the pressure supply unit, wherein a switching valve of a particular wheel brake with the lowest nominal pressure remains open until the nominal pressure has been set in the particular wheel brake; and wherein, for the switching valves of the wheel brakes not having the lowest nominal pressure, the control device is configured to individually calculate temporal durations for which the switching valves of the wheel brakes not having the lowest nominal pressure are to remain open so that the pressures in the respective wheel brakes are reduced to respective nominal pressures, and to open the switching valves only for the respective calculated temporal durations.
 19. The brake system according to claim 18, wherein the switching valve of the particular wheel brake is opened first at a time at which pressure in an associated brake circuit is equal to or higher than an actual pressure in the particular wheel brake.
 20. The brake system according to claim 19, wherein a piston of the pressure supply unit separates a first working chamber and a second working chamber tightly from each other, wherein both the first and the second working chamber are enabled, by corresponding actuation of isolating valves, to be used for simultaneous and/or temporally offset pressure build-up and pressure reduction in the wheel brakes of the brake circuits.
 21. The brake system according to claim 20, wherein the at least one isolating valve is designed for high throughflow and low differential pressure.
 22. The brake system according to claim 21, wherein, by means of the pressure supply device before or during opening of an isolating valve of the at least one isolating valve, the control unit is configured to set a pressure in a corresponding pressure chamber of the pressure supply unit by a movement of the piston of the pressure supply unit that is so great that a sufficiently small differential pressure is created at the isolating valve to be opened.
 23. A brake system for motor vehicles, the brake system including: an actuation device, a travel simulator configured to generate a feedback force on the actuation device, a first piston-cylinder unit, having at least one piston that separates two working chambers from each other, wherein each working chamber is connected via at least one hydraulic connecting line to at least one wheel brake of a brake circuit, wherein at least one wheel brake is assigned to each brake circuit and each wheel brake is enabled to be connected to an associated one of the hydraulic connecting lines via an associated controllable switching valve, a control device, at least one pressure supply unit configured to be driven by an electric motor and having at least one working chamber, wherein the at least one pressure supply unit is configured to build up and/or reduce brake pressure in one or more of the wheel brakes simultaneously or successively, and at least one outlet valve, wherein a respective one of the at least one outlet valve is associated only with a single wheel brake or is associated with one wheel brake of a respective brake circuit, wherein the outlet valve is arranged in a hydraulic connection between the wheel brake with which it is associated and a pressure medium storage container, wherein no further valve is arranged between the respective one of the at least one outlet valve and the pressure medium storage container, and wherein the control device has a memory configured to store an actual brake pressure set in each wheel brake and/or probable wheel brake pressures of each wheel brake continuously calculated and updated with measured values in a control model, wherein for simultaneous pressure build-up in at least two wheel brakes of one brake circuit, to different nominal pressures in the wheel brakes, the control unit is configured to evaluate a respective pressure-volume curve of each of the wheel brakes concerned and to use a highest nominal pressure to be generated to calculate a piston travel of a piston of the pressure supply unit required for this and to move the piston of the pressure supply unit by the piston travel, using the electric motor of the pressure supply unit, wherein a switching valve of a particular wheel brake with a highest nominal pressure remains open until the nominal pressure has been set in the particular wheel brake; and wherein, for the switching valves of the wheel brakes not having the highest nominal pressure, the control device is configured to individually calculate temporal durations for which the switching valves of the wheel brakes not having the highest nominal pressure are to remain open so that the pressure in the respective wheel brakes not having the highest nominal pressure is built up to respective nominal pressures, and to open the respective switching valves only for the respective calculated temporal durations.
 24. The brake system according to claim 23, wherein the switching valve of the particular wheel brake is opened first at a time at which pressure in an associated brake circuit is equal to or lower than an actual pressure in the particular wheel brake.
 25. The brake system according to claim 23, wherein at the same time as, or temporally overlapping, the pressure build-up in the wheel brakes over the brake circuit(s), the pressure is reduced in at least one further wheel brake by opening of an associated outlet valve.
 26. The brake system according to claim 23, wherein the pressure supply unit is a piston-cylinder system having a single piston, wherein the single piston is driven by the electric motor and delimits at least one working chamber. 